My understandings on Sessions in Tensorflow still seem to be flawed even after reading the official documentation and this tutorial.
In particular, does tf.global_variable_initializer() initialize global variables with regard to a particular session, or for all the sessions in the program? Are there ways to "uninitialize" a variable in / during a session?
Can a tf.variable be used in multiple sessions? The answer seems to be yes (e.g. the following code), but then are there good cases where we want multiple sessions in a program, instead of a single one?
#!/usr/bin/env python
import tensorflow as tf
def main():
x = tf.constant(0.)
with tf.Session() as sess:
print(sess.run(x))
with tf.Session() as sess:
print(sess.run(x))
if __name__ == '__main__':
main()
In particular, does tf.global_variable_initializer() initialize global variables with regard to a particular session, or for all the sessions in the program?
With regards to a particular session. Check this out.
tf.reset_default_graph()
x = tf.Variable(tf.random.normal([1,5]))
with tf.Session() as sess:
sess.run(tf.global_variables_initializer())
first_sess_out = sess.run(x)
with tf.Session() as sess:
sess.run(tf.global_variables_initializer())
second_sess_out = sess.run(x)
np.testing.assert_array_equal(first_sess_out, second_sess_out)
The assertion fails so it is per session.
Are there ways to "uninitialize" a variable in / during a session?
tf.reset_default_graph()
x = tf.Variable(tf.random.normal([1,5]))
with tf.Session() as sess:
sess.run(tf.global_variables_initializer())
first_init_out = sess.run(x)
sess.run(tf.global_variables_initializer())
second_init_out = sess.run(x)
np.testing.assert_array_equal(first_init_out, second_init_out)
Apparently there is, after running tf.global_variables_initializer() the variables got reinitialized. Thus, the assertion fails.
Can a tf.Variable be used in multiple sessions? The answer seems to be yes (e.g. the following code), but then are there good cases where we want multiple sessions in a program, instead of a single one?
Yes, it can be used as you can see on the examples above. Good cases are when you want to execute the graph multiple times in a single run.
Related
I have seen and tried two methods but could not understand what difference does it makes. Here are the two methods I used:
Method 1:
saver = tf.train.import_meta_graph(tf.train.latest_checkpoint(model_path)+".meta")
sess = tf.Session()
sess.run(tf.global_variables_initializer())
sess.run(tf.local_variables_initializer())
if(tf.train.checkpoint_exists(tf.train.latest_checkpoint(model_path))):
saver.restore(sess, tf.train.latest_checkpoint(model_path))
print(tf.train.latest_checkpoint(model_path) + "Session Loaded for Testing")
Method 2:
saver = tf.train.Saver()
sess =tf.Session()
sess.run(tf.global_variables_initializer())
if(tf.train.checkpoint_exists(tf.train.latest_checkpoint(model_path))):
saver.restore(sess, tf.train.latest_checkpoint(model_path))
print(tf.train.latest_checkpoint(model_path) + "Session Loaded for Testing")
What I want to know is:
What is the difference between the above two methods?
Which is the best method to load the model?
Please let me know what is your suggestions on this.
I will try to be as concise as possible so here are my 2 cents on the matter. I will comment on the important lines of your code to point out what I think.
# Importing the meta graph is same as building the same graph from scratch
# creating the same variables, creating the same placeholders and ect.
# Basically you are only importing the graph definition
saver = tf.train.import_meta_graph(tf.train.latest_checkpoint(model_path)+".meta")
sess = tf.Session()
# Absolutely no need to initialize the variables here. They will be initialized
# when the you restore the learned variables.
sess.run(tf.global_variables_initializer())
sess.run(tf.local_variables_initializer())
if(tf.train.checkpoint_exists(tf.train.latest_checkpoint(model_path))):
saver.restore(sess, tf.train.latest_checkpoint(model_path))
print(tf.train.latest_checkpoint(model_path) + "Session Loaded for Testing")
As for the second method:
# You can't create a saver object like this, you will get an error "No variables to save", which is true.
# You haven't created any variables. The workaround for doing this is:
# saver = tf.train.Saver(defer_build=True) and then after building the graph
# ....Graph building code goes here....
# saver.build()
saver = tf.train.Saver()
sess = tf.Session()
# Absolutely no need to initialize the variables here. They will be initialized
# when the you restore the learned variables.
sess.run(tf.global_variables_initializer())
if(tf.train.checkpoint_exists(tf.train.latest_checkpoint(model_path))):
saver.restore(sess, tf.train.latest_checkpoint(model_path))
print(tf.train.latest_checkpoint(model_path) + "Session Loaded for Testing")
So nothing wrong with the first approach but the second one is flat-out not correct. Don't get me wrong with this, but I don't like either of them. However, this is just a personal taste. What I want to do on the other hand, is the following:
# Have a class that creates the model and instantiate an object of that class
my_trained_model = MyModel()
# This is basically the same as what you are doing with
# saver = tf.train.import_meta_graph(tf.train.latest_checkpoint(model_path)+".meta")
# Then, once I have the graph build, I will create a saver object
saver = tf.train.Saver()
# Then I will create a session
with tf.Session() as sess:
# Restore the trained variables here
saver.restore(sess, model_checkpoint_path)
# Now I can do whatever I want with the my_trained_model object
I hope that this will be helpful for you.
I'm trying to understand how a with block behaves in the above scenarios. I assume to always have one graph and one session only. I understand that I have (at least?) 2 ways to work using a session in a with block:
Example 1 : using as_default() that creates a context manager:
Same documentation says:
Use with the with keyword to specify that calls to
tf.Operation.run or tf.Tensor.eval should be executed in this
session.
# Create session
sess = tf.Session()
# Enter with block on a new context manager
with sess.as_default():
# Train: Following line should result calling tf.Operation.run
sess.run([optimizer, loss], feed_dict={x: x_train, y: y_train)
# Eval: Following line should result calling tf.Tensor.eval
sess.run([loss], feed_dict={x: x_eval, y: y_eval)
Example 2 : with block on session as stated in same documentation in a lower section:
Alternatively, you can use with tf.Session(): to create a session that
is automatically closed on exiting the context, including when an
uncaught exception is raised.
# Enter with block on session instead of Context Manager
with tf.Session() as sess:
# Train: Following line seems calling tf.Operation.run as per my test
sess.run([optimizer, loss], feed_dict={x: x_train, y: y_train)
# Eval: Following is unclear
sess.run([loss], feed_dict={x: x_eval, y: y_eval)
I would like to understand what is the correct usage as I see GitHub examples of both cases but of course without the results. In my tests both Example 1 and 2 works for training. For evaluation it seems there is a difference that I can't understand. It exceeds my knowledge to browse the Tensorflow source. Can someone please explain?
They do slightly different things, so each may or may not be correct depending on the usage. tf.Session.as_default() will just ensure that the session is set as the default ones, so calls to eval and run will use that session by default:
import tensorflow as tf
sess = tf.Session()
with sess.as_default():
print(sess is tf.get_default_session()) # True
However, as stated in the documentation, tf.Session.as_default() will not automatically close the session after the with block. If you want that, you can use the session itself as the context manager:
import tensorflow as tf
with tf.Session() as sess:
# Do something with the session
sess.run([]) # RuntimeError: Attempted to use a closed Session.
However, although (from my point of view) not clearly documented, using a session as context manager also makes it the default session.
import tensorflow as tf
with tf.Session() as sess:
print(sess is tf.get_default_session()) # True
What is the point of tf.Session.as_default(), then? Well, simply when you want to temporarily make a session the default one but you do not one to close it after that (in which case you should manually close it later, or use it as an outer context manager). This is probably most relevant when you have more than one open session. Consider the following case:
import tensorflow as tf
with tf.Session() as sess1, tf.Session() as sess2:
print(sess2 is tf.get_default_session()) # True
Here sess2 is the default because its context was added later (it can be considered to be "inner" to the context created by sess1). But now maybe you want to make sess1 the default for a while. However you can not use sess1 itself as context manager again:
import tensorflow as tf
with tf.Session() as sess1, tf.Session() as sess2:
# Do something with sess2
with sess1:
# RuntimeError: Session context managers are not re-entrant.
# Use `Session.as_default()` if you want to enter
# a session multiple times.
So you can switch between one and other default sessions with tf.Session.as_default():
import tensorflow as tf
with tf.Session() as sess1, tf.Session() as sess2:
with sess1.as_default():
# Do something with sess1
with sess2.as_default():
# Do something with sess2
# This is not really needed because sess2 was the default
# in the outer context but you can add it to be explicit
# Both sessions are closed at the end of the outer context
Of course, you can be extra explicit even with one session, if you want:
import tensorflow as tf
with tf.Session() as sess, sess.as_default():
# ...
Personally, I have never used tf.Session.as_default() in my actual code, but then again I have rarely needed to use multiple sessions, and I prefer to use tf.Session.run() instead of relying on the default session anyway, but that is mostly a matter of personal taste I suppose.
I am running a neural network program from shell. It is running right but it prints all the information besides my output which I really don't need. The detail is given in the picture attached.
I haven't written anything in my code to print this un-necessary information.
It looks like you might have log_device_placement turned on. You can turn this off by either removing it from the config in tf.Session() entirely or setting it to False.
# Example 1
sess = tf.Session(config=tf.ConfigProto(log_device_placement=False))
# Example 2
sess = tf.Session()
References:
Tensorflow - Using GPUs
Since updating to tensoflow version 1.0 which introduced the new Saver V2, tf does not delete old files any more with the 'max_to_keep' argument. This is a problem on my system since my models are pretty big but my free space is limited.
Using the dummy program below I end up with following files for every number from 1 to 10 while I only expect the last 3 (8,9,10) to actually be there.
testfile-1.data-00000-of-00001
testfile-1.index
testfile-1.meta
program:
import tensorflow as tf
a = tf.Variable(name='a', initial_value=0)
addops = a+1
saver = tf.train.Saver(max_to_keep=3)
config = tf.ConfigProto()
config.gpu_options.allow_growth = True
sess = tf.Session(config=config)
sess.run(tf.global_variables_initializer())
for i in range(10):
sess.run(addops)
save_path = saver.save(sess, 'testfile', global_step=i+1)
sess.close()
Is this just me or this a known bug?
What are possible problems which could lead to this misbehavior?
Is there any log or smth similar I could get more information from?
I can reproduce this. It seems to be a bug.
However the problem is gone once I save into a different location (different from the executed .py file path)
save_path = saver.save(sess, 'data/testfile', global_step=i+1)
TensorFlow has two ways to evaluate part of graph: Session.run on a list of variables and Tensor.eval. Is there a difference between these two?
If you have a Tensor t, calling t.eval() is equivalent to calling tf.get_default_session().run(t).
You can make a session the default as follows:
t = tf.constant(42.0)
sess = tf.Session()
with sess.as_default(): # or `with sess:` to close on exit
assert sess is tf.get_default_session()
assert t.eval() == sess.run(t)
The most important difference is that you can use sess.run() to fetch the values of many tensors in the same step:
t = tf.constant(42.0)
u = tf.constant(37.0)
tu = tf.mul(t, u)
ut = tf.mul(u, t)
with sess.as_default():
tu.eval() # runs one step
ut.eval() # runs one step
sess.run([tu, ut]) # evaluates both tensors in a single step
Note that each call to eval and run will execute the whole graph from scratch. To cache the result of a computation, assign it to a tf.Variable.
The FAQ session on tensor flow has an answer to exactly the same question. I will just go ahead and leave it here:
If t is a Tensor object, t.eval() is shorthand for sess.run(t) (where sess is the current default session. The two following snippets of code are equivalent:
sess = tf.Session()
c = tf.constant(5.0)
print sess.run(c)
c = tf.constant(5.0)
with tf.Session():
print c.eval()
In the second example, the session acts as a context manager, which has the effect of installing it as the default session for the lifetime of the with block. The context manager approach can lead to more concise code for simple use cases (like unit tests); if your code deals with multiple graphs and sessions, it may be more straightforward to explicit calls to Session.run().
I'd recommend that you at least skim throughout the whole FAQ, as it might clarify a lot of things.
eval() can not handle the list object
tf.reset_default_graph()
a = tf.Variable(0.2, name="a")
b = tf.Variable(0.3, name="b")
z = tf.constant(0.0, name="z0")
for i in range(100):
z = a * tf.cos(z + i) + z * tf.sin(b - i)
grad = tf.gradients(z, [a, b])
init = tf.global_variables_initializer()
with tf.Session() as sess:
init.run()
print("z:", z.eval())
print("grad", grad.eval())
but Session.run() can
print("grad", sess.run(grad))
correct me if I am wrong
The most important thing to remember:
The only way to get a constant, variable (any result) from TenorFlow is the session.
Knowing this everything else is easy:
Both tf.Session.run() and tf.Tensor.eval() get results from the session where tf.Tensor.eval() is a shortcut for calling tf.get_default_session().run(t)
I would also outline the method tf.Operation.run() as in here:
After the graph has been launched in a session, an Operation can be executed by passing it to tf.Session.run(). op.run() is a shortcut for calling tf.get_default_session().run(op).
Tensorflow 2.x Compatible Answer: Converting mrry's code to Tensorflow 2.x (>= 2.0) for the benefit of the community.
!pip install tensorflow==2.1
import tensorflow as tf
tf.compat.v1.disable_eager_execution()
t = tf.constant(42.0)
sess = tf.compat.v1.Session()
with sess.as_default(): # or `with sess:` to close on exit
assert sess is tf.compat.v1.get_default_session()
assert t.eval() == sess.run(t)
#The most important difference is that you can use sess.run() to fetch the values of many tensors in the same step:
t = tf.constant(42.0)
u = tf.constant(37.0)
tu = tf.multiply(t, u)
ut = tf.multiply(u, t)
with sess.as_default():
tu.eval() # runs one step
ut.eval() # runs one step
sess.run([tu, ut]) # evaluates both tensors in a single step
In tensorflow you create graphs and pass values to that graph. Graph does all the hardwork and generate the output based on the configuration that you have made in the graph.
Now When you pass values to the graph then first you need to create a tensorflow session.
tf.Session()
Once session is initialized then you are supposed to use that session because all the variables and settings are now part of the session. So, there are two ways to pass external values to the graph so that graph accepts them. One is to call the .run() while you are using the session being executed.
Other way which is basically a shortcut to this is to use .eval(). I said shortcut because the full form of .eval() is
tf.get_default_session().run(values)
You can check that yourself.
At the place of values.eval() run tf.get_default_session().run(values). You must get the same behavior.
what eval is doing is using the default session and then executing run().